1. Field of the Invention
The present invention relates to a Coriolis mass flowmeter which measures mass flow utilizing Coriolis' force generated in proportion to the mass flow of a fluid flowing in the vibrating pipe to be measured.
2. Description of the Prior Art
Japanese Patent Application Laying-open No. 34683/1985, corresponding to U.S. patent aplication Ser. No. 818,475, file Jul. 25, 1977, discloses an example of such a type of mass flowmeter.
FIG. 9 is a schematic perspective view showing a flow detector of a conventional Coriolis mass flowmeter. As shown in FIG. 9, a flow detector 1 includes a housing 9, to which is attached a U-shaped cantilever measuring pipe 3 as being supported at both ends 31 and 32 thereof on the housing 9, having an open side (mid portion of the pipe). Between the both ends 31 and 32 of the measuring pipe 3 there is provided a cantilever resonator 120. The measuring pipe 3 and the resonator 120 have matched resonance frequencies so that they can be resonant with each other. A driver 41 made of a coil or magnet is provided between the bottom portion 33 of the measuring pipe 3 and the tip 121 of the resonator 120. A driving circuit 130 is connected to and actuates the driver 41 so that the U-shaped measuring pipe 3 and the resonator 120 can be driven or vibrated at their resonant frequencies. Sensors 42 and 43 detect displacement of the bottom portion 33 of the U-shaped measuring pipe 3 at both sides 33a and 33b thereof where two straight portions or brackets of the measuring pipe 3 translate into the bottom portion 33. The sensors 42 and 43 may be speed sensors made of coils and magnets, respectively. Output signals from the sensors 42 and 43 are input to a signal processing circuit 140 and converted therein into flow rate signals. A fluid to be measured flows in into the cantilever U-shaped measuring pipe 3 through an inlet port a from a communication pipe (not shown). The fluid which has passed through the measuring pipe 3 flows out through an outlet port b to a communication pipe (not shown).
Let us consider a case where the flow rate of a fluid is zero. It is assumed that the U-shaped measuring pipe 3 and the resonator 120 are vibrated by the driver 41 and the driving circuit 120 at resonance frequencies. In this case, output signals with no phase difference are obtained from displacement sensors 42 and 43, which move in the same manner.
When the fluid flows, a Coriolis' force is generated in a direction perpendicular to the direction of velocity of the fluid flowing in the vibrating measuring pipe 3. Since the fluid flows in opposite directions at both ends of the U-shaped measuring pipe 3, the direction of the Coriolis' force generated at one end is opposite to that at the other end, thus giving rise to a momentum around the axis O--O in the measuring pipe 3. In other words, a torsional vibration around the axis O--O parallel of the brackets of the pipe 3 is superimposed on a deflection vibration around the axis W-W perpendicular to the axis O--O. As a result, the displacement sensors 42 and 43 issue respective outputs which can be detected with a phase difference therebetween.
Since Coriolis' force is in proportion to the mass flow rate of the fluid, the phase difference (time difference) between the output signals from the displacement sensors 42 and 43 is proportional to the mass flow rate. Hence, measurement of the phase difference (time difference) between the output signals results in measurement of the mass flow rate of the fluid.
On the other hand, the resonance frequency of the measuring pipe 3 depends on its mass (which includes the mass of the fluid in the pipe) and rigidity. In this case, the rigidity of the pipe 3 is constant and does not vary, and the density of the measuring pipe 3, which varies depending on the variation of mass, is very small, and therefore, the resonance frequency varies depending on the variation in density of the fluid to be measured in the measuring pipe 3. Accordingly, measurement of resonance frequency gives measurement of density of the fluid to be measured.
However, it is required for a Coriolis mass flowmeter to, on one hand, reduce driving power of the measuring pipe in order to minimize power consumption, and, on the other hand, stabilize the vibration of the measuring pipe in order to perform measurements of flow rate and density of a fluid with high precision. In other words, it is necessary to maintain mechanical Q of the measuring pipe at a high level. In order to maintain the mechanical Q at a high level, an effective measure is to make the part fixing the measuring pipe to have a large enough size or provide the measuring pipe with a resonator which vibrates like a tuning fork. Increasing the size of the part fixing the measuring pipe is impractical since the flowmeter becomes too heavy. On the other hand, provision of a resonator is disadvantageous in that upon variation of the density of a fluid to be measured, the resonance frequency of the measuring pipe also varies, giving rise to difference in resonance frequency between the resonator and the pipe, resulting in failure to obtain a stabilized vibration or a high enough mechanical Q.
The complicated construction of a U-shaped measuring pipe as shown in FIG. 9 has another disadvantage in that when a bubble is formed or introduced in the pipe, it tends to remain in the pipe and it is rather difficult to remove it. This adversely influences the precision of measurement, or makes washing of the measuring pipe difficult, leading to the occurrence of pressure loss.